Graphene oxide quantum dots (GOQDs), a composite nanomaterial formed by the GOQDs with graphene and/or graphene-like structure(s), and methods for preparing them are provided. The method for preparing GOQDs comprises providing a carbon-based 3D bulk material having a graphite layer structure as an anode, positioning an end face of the material in contact and parallel with the liquid surface of an electrolyte solution, and intermittently or continuously cutting and exfoliating graphite layers at the end face by electrochemical oxidation to obtain GOQDs. The method for preparing the composite nanomaterial comprises adding powder comprising graphite and/or graphite-like layer structure(s) to a solution of GOQDs; under an assistant mechanical action of high shearing force, subjecting the GOQDs adsorbed onto the graphite and/or graphite-like layer structure(s) in the solution to a cycle of exfoliation, re-adsorption, and re-exfoliation, to obtain a composite nanomaterial formed by quasi-zero and/or quasi-two dimensional graphene and/or graphene-like structure(s) and GOQDs.